Originally Posted by
NorthShorePeaks
I ran into this exact situation in October and upgraded at the airport using the check in kiosk. They did not credit me 150% of miles only 100%. I realized afterward that there is fine print on this.
You are correct, what you have purchased was LMU (Last Minute Upgrade). That is booked into R class and mileage is 100% (T&C remains the same conditions to what you have booked)
OP is asking for a change of booking class, which requires reservations/ticketing to be involved. Different methods of buying into the same cabin.
If I recall, they charged me same day change fee at the airport (and not Anytime change fee)... But if you deal with a nice enough call center agent (which happened to me twice) they can waive the change fee altogether and just pay fare difference.
Like others mentioned, if you are 500 miles short, why not change the routing instead?